1. Field of the Invention
The present invention relates to a diaphragm apparatus of a lens for a CCTV camera.
2. Description of the Related Art
In a conventional diaphragm apparatus of a lens for a CCTV camera in which a stop of a lens is adjusted by a galvanometer (servo meter) driven diaphragm, an ND (Neutral Density) filter is attached to a diaphragm blade to extend the controllable range of the quantity of light. Since the drive torque of the galvanometer is relatively small, a light and resin ND filter is used.
Conventionally, a film filter for photographic use is utilized as a resin ND (Neutral Density) filter whose spectral transmittance characteristics are such that the transmittance is constant (flat) in the visible light region. The spectral transmittance characteristics of a known ND filter are shown in FIG. 3, by way of example.
However, a conventional ND filter permits infrared radiation to pass therethrough. Therefore, in a monochrome CCTV camera, the F-number is insufficient to restrict the quantity of light, and the spectral distribution is such that the transmittance of infrared radiation is greater than that of the visible light, so that the object portion which exhibits high reflectivity to infrared radiation is made intensive (bright). Consequently, the picture looks like infrared photography and is very hard to see.
Moreover, in recent years, even in color CCTV cameras, an infrared filter which does not permit infrared radiation to pass therethrough is removed when a picture is taken, in order to make the image visible when it is dark (nighttime), even though the color reproduction is slightly attenuated when it is light (daytime). However, removal of the infrared filter makes it impossible to adjust the quantity of infrared radiation transmission.